1. Field of the Invention
The present invention relates to a method of making a lens with a first convex lens surface and a second lens surface, especially for a motor vehicle headlight, in which a glass melt drop is supplied to a concave mold and both sides of the glass melt drop in the mold are precise pressed together with a press die. The invention also relates to an apparatus or tool for precise pressing both sides of the lens.
2. Related Art
Many lenses, especially those for motor vehicle headlights, do not need to fulfill the highest specifications, like those for objectives in optical lenses. On the other hand, the dimensions of the lenses that must be taken into account for the assembly must not vary. Furthermore the lenses must be economical to manufacture.
Usually those lenses are made by a single-side precise pressing method, i.e. a blank is heated, the aspheric side is pressed and the flat or planar side is after-worked by grinding and polishing. The technique for manufacturing single-side precise pressed optical components for the main motor vehicle headlight has matured, but is not suitable for mass production with a high productivity. This production technology is organized into a number of manufacturing steps, which occur during manufacturing according to the following sequence:
A. Batch preparation
B. Melting
C. Hot forming
D. Tempered cooling of the semi-finished product
E. Quality control
F. Packaging and delivery to the end user
1. Heating again
2. Pressing (one side to final shape)
3. Tempered Cooling
4. Grinding (second side plane)
5. Polishing (second side plane)
6. Faceting (edges, due to break off)
7. Packaging
It has been shown that the feeder provides no exactly constant drop weight. The variation of the drop weight, which is from 100 to 200 g, is up to ±2%. Volume shrinkage occurs during the pressing process. These variations must not impair the installed dimensions of the formed lens.
A method for precise pressing optical components is known from DE 100 43 065 A1, in which excess material can escape into an unlimited free space, which is present between the lower and the upper mold part. A concentric ring arises on the glass part, which does not act on the optical characteristics of the precise pressed glass part. Differences in volume of the proportioned glass part can be balanced or compensated by different widths of the rim without problem. The width of the rim of the precise pressed glass part has hardly any effect on the optical functioning of the lens in contrast to changes in the thickness of the rim. However the rim does not protrude from the plane of the lens surface. A completely similar method, in which however a lens is made with two curved lens surfaces, is described in U.S. Pat. No. 4,698,089 A.